Note that this MitM attack wouldn't work generally, (in the way that the government is saying they can), with other secure messaging--- it just happens that the way Apple is implementing it with their imessaging, is vulnerable in the way the feds, (and others here), are describing, to MitM.
There are probably plenty of other apps out there that wouldn't be vulnerable to this MitM, but they aren't as popular as imessaging.
